1. Field of the Invention
The present invention generally relates to the field of melt extraction wherein solidifying filaments are extracted from a source of molten material by means of a rotating heat-extracting member, such as a chill wheel or disk. More particularly, the invention involves melt extraction wherein the molten material is fed to the chill wheel through an orifice or nozzle.
2. Description of the Prior Art
The basic technique of extracting continuous or discontinuous filaments of controlled length from a source of molten material by contacting the latter with a rotating heat-extracting member is well documented in the prior art. This procedure involves utilizing a chill wheel or disk having a peripheral surface which, when rotated against the surface of the molten material, immediately solidifies and removes a filamentary product of the material on the peripheral surface, with the product being thereafter spontaneously released by the rotating member.
It is known to extract filaments by rotating chill wheels or disks directly against the surface of an open bath of molten material, such as molten metal. This is known as the open bath technique of melt extraction and is inherently difficult to control for several reasons. First, the rotating action of the chill wheel causes a pumping action within the molten bath which in turn causes fluid turbulence which, if large enough, causes premature termination of the extraction process. This turbulence increases with the rotational velocity of the wheel and also causes nonuniform product configuration and structure. Another problem attendant the open bath technique involves the wide exposure of the molten metal to any atmosphere which causes oxidation products to accumulate around the critical extraction area, thereby causing the incorporation of oxide impurities into the solidified product. Examples of the open bath technique for melt extraction and proposals for overcoming the problems associated therewith are disclosed by the Mobley U.S. Pat. No. 3,861,450; Bedell et al. U.S. Pat. No. 3,863,700 and Maringer et al. U.S. Pat. No. 3,094,344.
Another fundamental technique utilized in the practice of melt extraction involves the feeding of the molten material through an orifice or nozzle so that the problems inherent with the open bath technique can be minimized and the size and shape of the filamentary product can be better controlled. This is known as the orifice technique and utilizes a rotating chill wheel which pulls the molten material directly from the orifice or mouth of the nozzle, thereby minimizing fluid turbulence and the exposed surface area of the melt. The rate of production of a filament by the orifice technique is dependent not only upon the rotational velocity of the chill wheel, but also on the rate at which the molten material is fed through the orifice. The basic orifice technique is well exemplified in early disclosures, particularly Strange et al. U.S. Pat. No. 905,758; Strange et al. British Pat. No. 155,548 (1913); and Strange British Pat. No. 24,320 (1909).
While the orifice technique does provide fundamental advantages over the open bath technique of melt extraction, the former technique nevertheless is also confronted with potential problems and considerations that may hinder continuous melt extraction and precise product control. Correlating the rotational velocity of the chill wheel with the molten material feed rate from the orifice is critical in assuring a product of uniform configuration and thickness. The orifice itself must be defined by an aperture formed by a solid material that has sufficient resistance to degradation under high heat conditions, particularly those required for extracting high melting point metals. Control of product quality is also dependent upon the actual shape or configuration of the orifice and its disposition with respect to the rotating chill wheel. Despite the small exposed surface area afforded by the orifice technique, the undesirable formation of oxides is still possible when molten metals are being extracted.
Prior art efforts to overcome some of the problems confronting the orifice technique of melt extraction have included the Maringer et al U.S. Pat. No. 3,896,203 which, in one embodiment of the invention disclosed thereby, utilizes an inverted container having a small orifice at its lowermost end to provide a controlled gravity feed of molten metal through the maintenance of a stable pendant drop of the metal at the orifice for contact by the outer circumferential edge of a rotating disk-like heat extracting member. The Strange British Pat. No. 20,518 (1910) and King U.S. Pat. No. 3,522,836 both propose the broad concept of extracting molten metal from a meniscus formed at the exit orifice of a feed nozzle.
The heretofore prior art efforts at melt extraction through the orifice technique have not generally resulted in consistent product quality. This is particularly evident when strips or ribbons of metal are being extracted. Because of the obvious economical advantages derived from making metal products, such as containers, from metal strip stock requiring a minimum of mechanical treatment, it is extremely desirable that high quality metal strips be produced through melt extraction. Moreover, there also exists a great need for both efficient and economical production of metal strips having almost any desired thickness or composite structure through melt extraction according to the orifice technique.